Today the 2nd of June, 2021, I want to give my own contribution on the above topic/question.
According to the bible, it is not a good practice for two unmarried people to sleep together. The bible calls it fornication or sexual immorality.
1 Thessalonians 4:3-4 ESV
For this is the will of God, your sanctification: that you abstain from sexual immorality; that each one of you know how to control his own body in holiness and honor,
I know we live in a time so many things are no longer seen as serious as they use to be. Sexual practices in our days is something so common among young people these days. They see see it as a thing of fun and play but the bible is against it.
We need to get it right that the act of sex or sexual practices has been defined within the confinement of marriage.
You will discover that one of the fruits of the Holy spirit is self control but where you cannot control yourself, marry and that's of to them that are of age.
In conclusion, sleeping together before marriage is a sin and as a child of God, you should abstain from sexual immorality.
The only sin described in the bible as sin against one's body which is the temple of the holy spirit.